The invention relates to a magnetic bearing device for a turbo-molecular pump and a machining device.
A turbo-molecular pump or a machining tool requires high speed rotation with low vibration, and a magnetic bearing device is provided as a bearing. In general, a magnetic bearing device of a five-axis control type has been used for a rotating member, in which four axes are disposed in a radial direction and one axis is disposed in an axial direction. Each axis is provided with an electromagnet and a displacement sensor as a unit. The displacement sensor typically includes an inductance type sensor for detecting a displacement of a rotating member. In many cases, an opposed type sensor has been disposed in the radial direction, and a non-opposed type sensor has been disposed in the axial direction. A carrier wave is applied to the displacement sensors, and the carrier wave is modulated in amplitude according to a change in impedance of the sensor units. The modulated signal is used for controlling an excitation current of the electromagnets.
The displacement sensor disposed in the axial direction is not the opposed type, so that an offset value of the sensor signal tends to fluctuate due to a change in floating capacitance of the sensor unit or a signal line. In particular, when a cable between the magnetic bearing device and a controller is replaced, the offset value tends to fluctuate greatly due to a change in floating capacitance of the cable. Accordingly, it is necessary to adjust the offset value every time when the cable is replaced. In order to eliminate the adjustment of the offset value, the carrier wave tends to have a lower frequency.
The sensor signal is filtered to remove a basic wave and a harmonic component of the carrier wave from the modulated signal. When the carrier wave has a lower frequency, the frequency of the carrier wave becomes closer to a required frequency range of the sensor signal. Accordingly, it is necessary to eliminate a phase delay of the required frequency range and to use a notch filter instead of a low-pass filter, so that magnetic levitation is properly controlled. As a result, a circuit size tends to become large.
In view of the problems described above, an object of the present invention is to provide a magnetic bearing device capable of lowering a frequency of the carrier wave while reducing a circuit size.
Further objects and advantages of the invention will be apparent from the following description of the invention.